Voltage indicating and recording device



May 25, 1954 E. P. FELCH, JR., ETAL 2,579,630

VOLTAGE 4INDICATING AND RECORDING DEVICE Filed Feb. 12, 1942 um n N ao, a/

GS FILLED v N IHHIIHH ummm AAAAAAAAA EIR FELCH, JR 7T SLONCZEWSK/ 5V A ORA/Er Patented May 25A, 1954 UNITED STATES PATENT OFFICE VOLTAGE INDICATING AND RECORDING DEVICE New York devices for indicating and recording electrical phenomena such as, for example, low intensity voltages.

The problem frequently arises of properly and quickly indicating and recording the intensity of an alternating voltage which may at times have its phase changed by 180 degrees, and oi' indicating which of the two phase conditions is present. Such a problem arises, for example, when it is desiredto record the magnitude and phase condition of a voltage induced in a search coil which is adapted to move through a magnetic field the intensity of which at any point may be different than the intensity at adjacent points and the direction of which at every point is one of two directions, 180 degrees apart.

It is a primary object of the present invention to perform one or more of the operations of indicating and recording the intensity of a voltage which may at times have its phase changed by 180 degrees and also indicate which of the two phase conditions is present.

It is another object of this invention to provide a recording instrument in which apen or similar marking element has a positive, quickacting mechanism for driving it in either direction from a central position and wherein mev chanical inertia effects are greatly reduced, re-

sulting in an increased speed of response with improved stability and reliability.

It is a further object of this invention to prof vide novel and improved apparatus for varying a voltage to obtain a balance between it and a second variable voltage.

The foregoing and related objects are attained in an illustrative embodiment of the invention by providing a novel circuit wherein an alternating voltage, which can be varied in intensity and phase condition, is connected in phase opposiknown voltage and the balancing voltage (the one which is varied to produce a balance) is the input circuit of an amplier across which is apy plied a voltage equal in magnitude to the vector Application February 12, 1942, Serial No. 430,540

(c1. sai- 99) 2 sum of the unknown voltage and the voltage utilized in balancing. The output circuit of the amplier is applied to a push-pull detector, the amplied voltage k being applied, preferably through a transformer, between the control elements of the two electron discharge devices, while another alternating voltage, of the same frequency as the amplied voltage and of such phase that it is added to the voltage (from the amplier) applied between the control element and the cathode of one of the tubes and is subtracted from the voltage (from the amplier) applied between the control element and the cathode of the other of said tubes, is applied in the connection between the common terminal of the two cathodes and the mid-tap of the secondary winding of the transformer. Inasmuch as a condenser is provided between the anode and the cathode of each of these two tubes to shunt out the alternating current in the output wave, there flows through resistances connected between the two anodes, a direct current which varies in direction dependent upon the phase condition of the vector sum voltage, or, expressed in other words, dependent upon the algebraic sign of the vector sum voltage. This direct current is applied to a motor driving circuit preferably comprising two gas-filled, grid-controlled devices of the "Thyratron type each having connected between its cathode and anode an armature of a motor. Each motor is adapted to drive a pen or stylus in a direction transverse to the direction in which the paper chart is driven by rollers, one of said motors when energized tending to drive the pen in one direction from a central position and the other of said motors adapted to drive the pen in the other direction from the central position. The direct current for the eld windings of the motors is supplied from any suitable source. An alternating Voltage is applied to the output circuit of each of said gas-lled devices in order that the grid or control element of each device may regain control of the discharge therein. The direction of flow of the direct current applied to the gas-filled devices input circuits determines which of the gas-filled devices discharges and thus determines which of the motors is driven. The motors also vary the position of the movable tap on a slide wire potentiometer (to produce the"balancing voltage) until balance is reached (the vector sum voltage is zero). The external terminals of vthe potentiometer are connected to a source of alternating voltage. Preferably this voltage is obtained from the same source which energizes the coils setting up the magnetic eld which is measured at various positions of the Search coil. This source may also supply the voltage which is applied to the common input circuit of the two electron discharge devices constituting the pushpull detector. A scale calibrated in positive and negative volts is preferably placed in front of the potentiometer to indicate the intensity and phase condition of the unknown voltage.

The invention will be more readily understood by referring to the following description taken in -connection with the accompanying drawings forming a part thereof in which:

Fig. 1 is a schematic diagram of a recorder of this invention and of a system wherein it may be used; and

Fig. 2 is a more detailed showing of a portion of the arrangement shown in Fig. l.

Referring more specifically to the drawing, Fig. 1 shows apparatus for indicating and recording a voltage, which by way of example to illustrate the principles of novelty of this invention, will be considered as the voltage induced across a search coil I0, which Voltage may have one or the other of two phase conditions 180 degrees apart, and for indicating which of these two phase conditions is present. While the voltage to be measured by the recording device of this invention is described as that existing across a search coil, it will be apparent that the invention is not limited thereto and the recorder is equally applicable to the measurement of any alternating voltage. As a specific example of an application of the recorder of this invention, there is shown in Fig. l a series of coils I I each of which receives current of, for example, 270 cycles, from a source I2 through a transformer I3 and switching apparatus I1. Each coil Il may, for example, comprise a continuous single layer of high resistance wire wound around a strip of phenol fibre about five feet long. Because -of this manner of construction, the resistance of each coil is very large compared with its inductance. The transformer I3 has its primary winding connected to the generator I2 and its secondary winding `I5 tapped to provide many inner terminals. As a specific eX- ample, the mid-point I6 of the secondary winding I is connected to ground and there are also provided 100 taps above this point and 100 taps below this point. secondary winding I5 of the transformer I3 are connected to suitable switching apparatus represented by the box I1 from which output leads are connected to each of the coils I I. One terminal of each of the coils I I is connected to ground and the other terminal of each coil is connected by means of the switching apparatus I1 to any one of the 100 positive voltages (at a certain instant) or the 100 negative voltages (at this same instant) provided by the transformer I3. It will be apparent that if at any given time a positive potential is applied to the ungrounded terminal of any one of the -coils II, the current at that time flows therein in one direction while if a negative potential is at that time applied to the ungrounded terminal of this'coil, ra current hows therein in the opposite direction. The search coil I0 is adapted to move in various planes under the lower ends of the coils II. Three positions for such coils have been indicated by the full line search coil I0 and the dotted coils A and B. It will be apparent that, in anyposition, the flux linking the coil I0 is set up by current owing through many of the coils I I and as the current ows vin one direction through some of the coils All of the terminals of the and in the opposite direction through others of the coils, the voltage induced across the search coil IIJ may assume one phase condition at one position and have a phase condition degrees removed from this first-mentioned condition at some other point. The recorder of this invention, as pointed out above, not only indicates the voltage induced across the search coil i0 but also indicates which of the two phase conditions is present. Considering the portion of the field lying in a horizontal plane beneath the coils II, it is apparent, then, that each elemental portion thereof has an intensity and instantaneous direction along any particular horizontal line through that portion in said plane which depends respectively upon theamplitudes of the currents in the coils above and the instantaneous direction ofthe currents through those coils. Each current may be given either of two phase conditions, 180 degrees apart, with respect to their common source, depending upon which of the two possible ways of connecting the coil terminals to the source is at that time being utilized. Search coil Ii! may be moved in this plane so as to scan the field at this plane for all the elementary areas thereof. The electromotive forces induced in coil IG may in portions of this plane have their phase different by 180 degrees from that of the remaining ones in other portions of this plane. By trial (or by calculation where possible), the polarity of the connections of the coils and the intensity of the currents through the coils may be chosen so that the eld simulates on a reduced scale, approximately at least, another field, which field has previously been investigated along one plane (or other small number of planes) only, like the one mentioned above, to determine the field characteristics of the individual elemental areas thereof. The simulated iield so produced may then be investigated, by means of the search coil I0, in other planes as desired. Thus, a model of a large magnetic iield may be provided for labora tory investigation, which may be particularly useful in case the original eld is such that a desired investigation of it may be more easily made from a model.

The coils II and switching apparatus Il per se form no part of applicants invention. They have been shown merely to illustrate one possible use of the invention, which, as will appear from the appended claims, is not limited to any particular means for originating the electrometive forces from which those utilized are derived. Various uses of the invention in its diiferent aspects defined by the claims will occur to those skilled in the electrical arts.

The search coil I0 is connected in series with the input circuit of an amplier 20 and that portion of a potentiometer 2l connected between an inner terminal 22 and the effective mid-point or intermediate balance point of the potentiometer 2 I. This balance point may be grounded directly or it may be grounded externally by grounding the mid-point of the secondary winding 24 of a transformer 25, the primary winding 26 of which is connected through suitable phase shifting apparatus to the outer terminals of the secondary winding I5 of the transformer I 3. This phase shifting apparatus is necessary in order that the voltage between the balance point of the potentiometer 2l and the point 22 at which the slide wire finally comes to rest after balance is obtained is exactly equal to and opposite in phase with respect to the voltage induced in the search coil IIJ. Due to the fact thatthe vice 49.

l windings 42 and 43.

' across the windings 42 and 43.

shifting apparatus (not shown) may be provided,v if'necessary, to obtain this phase relationship.

1 coils il have very little inductance in comparison with their resistance, the voltage induced across the search coil Ill is substantially 90 degrees out of phase with respect to the voltage across the secondary winding |5. Consequently, the leads 21 and 28 from the secondary Winding |5 are connected to a series circuit comprising two condensers 29 and 30 and the resistance 3|, the constants of which are so chosen that the voltage across the resistance 3| is substantially in phase or 180 degrees out of phase with respect to the effective mid-point. By means now to be described, the voltage appearing in the input circuit of the amplifier is reduced to zero and at the same time (see Fig. 2) a pen, stylus or other indicating device 34 is driven `across a chart 35, which is adapted to be moved between an unwinding roll 32 and a winding roll (not shown), the pen or'other marking element 34 being moved in a direction transverse to the direction of the movement of the chart 35. An indicating scale 33, which is schematically indicated in Fig. 1 as being on the side of the sliding element 83y remote from the resistance 2|, is

preferably mounted -above the chart and in front of the resistance 2|. This scale is calibrated so that the central position is zero, plus volts (or millivolts) being indicated on one side of this zero position and minus volts (or millivolts) being indicated on the other side of this zero position. The plus and minus indica- Vtions tell the observer which of the two phase conditions of the search coil voltage is present.

amplier 2|! is alternating, it is necessary to obtain a direct current properly representative in direction of the phase of this alternating current to drive one or the other of the motors MI and M2. For this purpose the output current from the amplifier 2!) is applied to the primary winding 40 of the transformer 4| which has two secondary windings 42 and 43 having a common terminal 44. The outer terminal of the winding 42 is connected to the control element 45 of the electron discharge device 48 while the outer terminal of the winding 43 is connected to control element 5|) of the electron discharge de- The cathodes 46 and 5| of the tubes 48 and 49, respectively, are connected through secondary winding 53 and the transformer 54 to the common terminal 44 of the secondary The primary winding 55 of the transformer 54 is connected across the terminals of the primary winding 26 of the transformer 25 whereby the voltage induced in the secondary winding 53 is either in phase or 180 degrees out of phase with respect to the voltage Suitable phase Connected between the anode 41 and the cathode 46 of the tube 48 is the condenser 56 and between the anode 52 and the cathode 5| of the tube 49 is connected a similar condenser 51. The purpose of the condensers 56 and 51 is to provide a shunt path for the alternating current in the output circuit of the tubes 48 and 43. Resistors 59 and 60 are connected between the anode 41 and the anode 52 and the common terminal o1' these resistors is connected through a source of plate potential 58 to the common terminal of the cathodes 46 vand 5|. The tubes 48 and 49 and the circuit elements connected thereto are so selected and adjusted that in the absence of current in the secondary windings 42 and 43, the direct -current in resistor 59 is equal and opposite to the direct current in resistor 6U.

The anode 41 of the tube 48 is connected directly (or through resistor 96) to the control element 12 of a grid-controlled gas-filled electron discharge device 1|) while the anode 52 of the tube 49 is connected directly (or through resistor |11) to the control element 11 of a gaslled grid-controlled electron discharge device 15. Connected in circuit between the anode 13 and the cathode 1| of the device 1|! is the secondary coil 33 of the transformer 8| and the armature winding of the motor Ml. A condenser 14 is connected between the cathode and control element of the tube 10 while a similar condenser 19 is connected between the cathode and control element of tube 15. The condensers 14 and 19 and the resistors 915 and 91 are not absolutely essential but do aid somewhat in maintaining stability to prevent the tubes 13 and 15 from liring from extraneous transient currents. They must not, however, be so large in capacity as to slow down appreciably the response of these tubes to normal input signals. Connected in circuit between the anode 18 and the cathode 16 of the discharge device 15 is the secondary winding 82 of the transformer 8| and the armature winding of the motor M2. Alternating current of a frequency of, for example, 6|) cycles, is applied to the primary winding 83 of the transformer 8| from the source 84. Direct current for thevfield windings 35 and 86 of the motors MI and M2, respectively, is obtained from a suitable direct current source 81 or the iields may be of the permanent magnet type. The motor armatures are connected to a drive shaft 88 in such a way that the rotation of the motor Ml drives the drive shaft 38 in one direction with the consequent movement of the follower 89 in one transverse direction, while a rotation of the motor M2 rotates the shaft 88 in the opposite direction and moves the follower 89 in the opposite transverse direction. The movement of the follower 89 moves the pen or other marking element 34 in transverse direction across the chart 35 and at the same time varies the position of the variable tap 22 on the resistor 2| and moves the needle 95 across the indicating scale 33. kThe chart 35 is unwound from roller 32 and driven overroller 93, to a wind-up roller (not shown) as indicated schematically in Fig. 2. If the search coil lll is driven from one position to another by l motor means, it is preferable to synchronize this phase therewith.

'While two motors have been Vshown in the arrangement ofiFig. 1,.it is obvious that one motor With'reversible windings may be used instead.

The operation of the device shown in Fig. l

is as follows: Assume that the search coil it is lplaced as shown in the full line position in the drawing and there is induced thereacross a voitage which has a certain phase condition with respect to the voltage of the generator l2. With the connections as shown, this voltage will be substantially 90V degrees out of phase with respect connected to ground by grounding the mid-point of the secondary winding 24 of the transformer 25. Depending upon the position of the tap 22 before the motor apparatus acts to shift it, the potentiometer voltage in circuit with the voltage across the Search coil i is either in phase with the search coil voltage or 180 degrees out of In either case, however, the voltage applied to the input circuit of the amplier 2U is equal in magnitude to the vector sum of the search coil voltage and the voltage between the variable tap 22 and the effective midpoint of the transformer 2|. The amplifier 2li may be two or more stages and the output current thereof is applied to the primary winding 4U of the transformer 4|, the secondari7 winding of which is split so that half of the secondary voltage is applied between the cathode 46 and the control element 45 of the tube 48 and the other half (in opposite phase) is applied between the cathode and control element of the tube 49. By means of the transformer 54, a voltage is applied between the cathode and the control element of each of the tubes which is in phase with one and out of phase with respect to the other of the input voltages from the secondary windings 42 and 43. When no current iiows through the primary winding 40, equal input signals are applied to control elements 45 and and in the same phase so that the currents in the output circuit of these tubes 48 and 49 are balanced. The bias on the control elements of the gas tubes and l5 is made such that when this balanced condition is present, neither tube fires. Now, when a current flows through the winding 4U of the transformer 4|, the voltage across one-half of the spiit secondary 42, 43 is added to the voltage across the secondary winding 53 to produce a larger input signal through one of the tubes 48 and 49 while the voltage across the other of said Secondary windings 42 and 43 is subtracted from the voltage across the winding 53 to produce a smaller input signal for the other of these tubes. Thus one tube produces a greater current than the other. The alternating current in the output circuits of the tubes 48 and 49 is shunted by the condensers 56 and 51 and there appears across the resistors 59 and 6U a direct current the direction of which is dependent upon which of the tubes 4B and 49 is conducting the most current. When this current is in one direction, for example in a direction which will tend to make the control element 12 of the gas-filled device 1|) swing in a positive direction and which will make the control element 11 of the gas-filled device 15 swing in a negative direction, motor MI is actuated while motor M2 is not. The direct current flow will be in the opposite direction and motor M2 will be actuated while motor MI remains unoperated when the vector sum voltage has a phase condition which is 180 degrees removed from the iirst assumed phase condition. The gas-filled tubes 'l and 15 are normally (in the absence of current in the winding 40) biased to cut-off by means of the bias battery 9|. In fact, battery 9| is adjusted to slightly overbias these tubes so as to allow a slight margin to aid in maintaining stability. Assume that the phase relationship in the circuit just described is such that the direct current in the resistors 59 and 6|] is in such a direction that it causes the voltage of the grid of tube 10 `to increase in the positive direction beyond its critical control point and that of the grid of the gas-filled tube 15 to become more negative at the same instant that both plates 'i3 and 78 are positive. With these conditions itis obvious that plates 13 and 18 become positive sixty times per second in accordance with the variations in voltage supplied thereto from the source 84. It will be apparent under these conditions that only gas tube 10 will re since the grid of the tube 15 is 180 degrees out of phase with the voltage on the plate. This drives motor MI in a direction which tends to decrease the input voltage of the amplifier 20. As longas this current flows in the input circuit of the amplifier Eil, or in other words as long as that portion of potentiometer voltage 2| between the variable tap 22 and the effective mid-point does not balance the voltage across search coil if), tube 'Hi will continue to fire on every positive half cycle. Each time that tube 1D fires a positive pulse of current will flow through motor armature Mi and motor MI will very rapidly increase in speed, the actual final speed which it attains depending upon the amount of departure from balance. After a gas tube fires the grid loses control thereover but the use of the alternating voltage in the plate circuit of the tubes 'I0 and "i5 permits each grid to regain control by stopping the discharge once each cycle. So long as the direction of the direct current through resistors 59 and Gil is such as to fire tube 10, the tube 15 cannot re because the grid voltage is made even more negative than the critical firing voltage.

Now if the direction of the direct current through the resistors 59 and 60 is changed, the

` tube 'l5 is fired and the armature of the motor M2 is actuated, and the armature of the motor Mi remains unoperated. It should be noted that the actual speed of the motor is not necessarily a linear function of the degree of departure from balance. It is more accurately expressed as being a monotonically increasing function of the departure from balance. The important thing to note is that as the condition of balance is approached, the speed of the motor is rapidly brought to Zero. This insures a high degree of stability and with the inertia of the mechanical parts kept at a minimum, the speed of response is very high. It has been found by actual construction that the coasting period between alternate discharges in the tubes 70 and 15 can be made very short, resulting in very close speed control and freedom from overshooting. For a more complete description of a somewhat similar motor control means actuated by two gas-filled devices, reference is made to an application Serial No. 297,088, filed September 29, 1939, by W. J Means and T. Slonczewski and which issued as Patent 2,396,187 on March 5, 1946. As one motor or the other rotates the shaft 88, the slider 89 moves over the paper chart 35 which is caused to move downward in the plane of the drawing by means of the driver roll (not shown) and to continue its tracing until a point of balance is reached between the potentiometer voltage and the search coil voltage. At this point neither motor MI or M2 is actuated as there is no voltage across the input circuit of the amplier 2B. If desired, the voltage at this balance point may be read from the calibrated scale 33 which, as pointed out above, may be mounted in front of the resistor 2l. If now the search coil l is moved to another position, such as for example the dotted position A or position B indicated in Fig. lfthe process described above is repeated until balance is again reached between the Search coil and the potentiometer voltage.

In actual practice, suitable mechanical guide tracks are employed to conine the direction of travel of the stylus or other marking member 34 and the slider 39 to a straight line. These tracks have been omitted in these igures for the sake of clarity but any of the forms of guide tracks well known in the recorder art may be employed.

While the invention has been shown and described in connection with a recording and indicating instrument, it will be obvious to those skilled in the art that certain of the principles employed and the circuits shown and described are capable of use in other devices such as, for example, in control circuits. Other modifications may be made in the embodiment described without departing from the spirit of the invention, the scope of which is indicated in the appended claims.

What is claimed is:

1. In combination, a source of alternating current, means including a search coil moving in a magnetic eld set up by current from said source for deriving an alternating voltage, means for deriving a second alternating voltage of the same frequency as said rst alternating voltage, and means for varying said second alternating voltage until it is equal in magnitude and in phase opposition to said rst alternating voltage, said last-mentioned means comprising means for deriving an alternating Voltage representing the vector sum of said first and second alternating voltages, means for deriving from said vector sum voltage a direct voltage the direction of which is dependent on the sign of said vector sum voltage, and means for utilizing said direct voltage to vary said second alternating voltage until it is equal in magnitude and in phase opposition to said rst alternating voltage.

2. A source of alternating voltage, direct current generating means for setting up a direct current in its output when an alternating voltage is applied to its input, a circuit element connected to'the input of said direct current generating means, two separate means for deriving two alternating voltages of the same frequency from said source and connecting them in a single series circuit with said circuit element, means controlled by said direct current for bringing the vector sum of said derived voltages to zero value, and a third means for deriving alternating voltage of said frequency from said source and applying it to said input of said direct current generating means to cause said direct current generating means to generate direct current in one direction when the vector sum of said rst two alternating voltages has one algebraic sign and in the opposite direction when the vector sum of said two alternating voltages has the opposite algebraic sign.

3. In combination, an impedance element, means for deriving across said element an alternating voltage which has an unknown magnitude and phase, a resistance member having two outer terminals and a variable inner terminal, means for applying a source of potential of the same frequency as said unknown voltage to the outer terminals of said resistance member, means for Xing the potential of the mid-point of said resistance member, means for varying the position of said inner terminal so that it can be connected to said resistance member on either one side or the other of said mid-point, means for connecting said impedance element and that portion of the resistance member included between said mid-point and said variable terminal in such a way that a voltage equal to the vector sum of said two voltages is produced, and means for reducing said vector sum voltage to zero, said last-mentioned means including motor means for varying the position of said inner terminal.

4. In combination, an impedance element, means for deriving across said element an alternating voltage which has an unknown magnitude and phase, a resistance member having two outer terminals and a variable inner terminal, means for applying a source of potential of the same frequency as said unknown voltage to the outside terminals of said resistance member, means for effectively fixing the potential of the mid-point ofv said resistance member, means for varying the position of said variable inner terminal so that it can be connected to said resistance member on either one side or the other of said mid-point, means for connecting said impedance element and that portion of the resistance member included between said mid-point and said variable inner terminal in such a way that a voltage is produced equal to the vector sum of said voltages across the element and said portion of the resistance member, and means for reducing said vector sum voltage to zero, said last-mentioned means including means for deriving from said vector sum voltage a direct voltage, the direction of which is dependent on the sign of said vector sum voltage, and means for utilizing said direct voltage to vary the position of said variable inner terminal of said resistance until said vector sum becomes zero.

5. In combination, an impedance element, means for deriving across said impedance element an alternating voltage which has an unknown magnitude and phase, a resistance member having two outer terminals and a variable inner terminal, means for applying a source of potential of the same frequency as said unknown voltage to the outside terminals of said resistance member, means for fixing the potential of the mid-point of said resistance member, means for varying the position of said variable inner terminal so that it can beconnected to said resistance member on either one side or the other of said mid-point, means for connecting said impedance element and that portion of the resistance member included between said mid-point and said variable inner terminal in such a way that a `voltage `is produced equal to the vector sum of charge devices connected in push-pull', each of said discharge devices having an anode, a cathode, and a control element, means for applying between the control elements of said two electron discharge devices a second alternating voltage having its magnitude and phase determined by the vector sum voltage, means for applying a third alternating voltage between the cathode and the control element of each of said electron discharge devices, said third alternating voltage having a phase relationship with respect to said vector sum voltage such that it is either in phase or 18'0 degrees out of phase with respect to said voltage, means for by-passing the alternating portion of the output wave in said pushpull arrangement whereby a direct current is produced the direction of which is dependent upon the algebraic sign of the vector sum voltage, and means for utilizing this direct voltage to vary the position of the variable contact on said resistance member until said vector sum voltage is equal to Zero.

6. In combination, an impedance element, means for deriving across said element` an alternating voltage which has an unknown magnitude and phase, a resistance member having two outer terminals and a variable inner terminal, means for applying a source of potential of the same frequency as said unknown voltage to the outside terminals of said resistance member, means for fixing the potential of the mid-point of said resistance member, means for varying the position of said variable inner terminal so that it can be connected to said resistance member on either one side or the other of said mid-point, means for connecting said impedance element and that portion of the resistance member included between said mid-point and said variable inner terminal in such a way that a voltage is produced equal to the vector sum of the voltages across the element and said portion of the resistance member, and means for reducing said vector sum voltage to zero, said last-mentioned means including a balanced detector arrangement comprising two electron discharge devices connected in pushpull, each of said discharge devices having an anode, a cathode, and a control element, means for applying between the control elements of said two electron discharge devices a second alternating voltage having its magnitude and phase determined by the vector sum voltage, means for applying a third alternating voltage between the cathode and control element of each of said electron discharge devices, said third alternating voltage having a phase relationship such that it is either in-phase or one-half wave-length outof-phase with respect to said vector sum voltage,v

means for by-passing the alternating portion oi' the output Wave in said push-pull arrangement whereby a direct current is produced the direction of which is dependent upon the algebraic sign of the vector sum voltage, two motors, one of them connected to move said variable inner terminal in one direction and the other of which is connected to move said variable inner terminal in the opposite direction, and means responsive to the direction of said direct current for energizing one or the other of said motors to vary the position of said variable inner terminal.

7. In combination, an impedance element,

means for deriving across said element an alternating voltage lwhich has an unknown magnitude and phase, a resistance member having two outer terminals and a Variable inner terminal, means for applying a source of potential of the same frequency as said unknown voltage to the outside terminals of said resistance member, means for fixing the potential of the mid-point of said resistance member, means for varying the position of said variable inner terminal so that it can be connected to said resistance member on either one side or the other of said mid-point, means for connecting said impedance element and that portion of the resistance member included between said mid-point and said variable inner terminal in such a way that a voltage is produced equal to the vector sum of the voltages across the element and said portion of the resistance member, and means for reducing said vector sum voltage to zero, said last-mentioned means including a balanced detector arrangement comprising two electron discharge devices connected in push-pull, each of said discharge devices having an anode, a cathode, and a control element, means for applying between the control elements of said two electron discharge devices a second alternating voltage having its magnitude and phase determined by the Vector sum voltage, means for applying a third alternating voltage between the cathode and control element of each of said electron discharge devices, said third alternating voltage having a phase relationship such that it is either in phase or 180 degrees out of phase with respect to said voltage, means for lay-passing the alternating portion of the output wave of said push-pull arrangement whereby a direct current is produced the direction of which is dependent upon the algebraic sign of the vector sum voltage, two motors, one of which is connected to move said variable inner terminal in one direction and the other of which is connected to move said variable inner terminal in the other direction with respect to said mid-point, and means responsive to the direction of said direct current for energizing one or the other of said motors to move said variable inner terminal, said last-mentioned means comprising two gas-lled grid controlled electron discharge devices, means for connecting the armature of one of said motors in the circuit between the cathode and the anode of one of said gas-lled devices and means for connecting the armature of the second of said motors in the circuit between the cathode and the anode of the second of said gas-lled devices, means for applying said direct voltage between the two grids, and means for applying an alternating voltage between said two anodes.

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